Check out the website http://grimeton.org/ to find out on the only radio station that is left in the once transatlantic network of nine long wave stations that were built during the years 1918-1924. Although not particularly useful for thickness sensing, the six 127-meter high antennas are a master piece of engineering. The reason why these antennas are so big? -> They operate a very low frequency of only 17 kHz, which is more than three million times 'slower' than the millimeter waves of Hammer-IMS. Different frequencies mean different antennas...